Why don't xUnit frameworks allow tests to run in parallel?

Question

Do you know of any xUnit framework that allows to run tests in parallel, to make use of multiple cores in today's machine?

If none (or so few) of them does it, maybe there is a reason... Is it that tests are usually so quick that people simply don't feel the need to paralellize them?

Is there something deeper that precludes distributing (at least some of) the tests over multiple threads?